Radiological Mimickers of COVID-19 Pneumonia: A Pictorial Review

Computed tomography (CT) scan plays an important role in the early diagnosis of coronavirus disease (COVID-19) pneumonia. In resource-limited regions with limited availability of polymerase chain reaction (PCR) kits, CT findings, together with appropriate clinical parameters, are used to establish an accurate diagnosis. However, since the radiological findings are non-specific, the CT features may overlap with the findings of several other categories of pulmonary diseases. Diagnosis based on radiological features can be especially challenging in the presence of a comorbid lung disease. This study aimed to describe the radiological findings of a wide spectrum of lung pathologies, with emphasis on their similarities with the common presentations of COVID-19 pneumonia.


INTRODUCTION
Since the emergence of a novel coronavirus disease,   This study aimed to discuss and illustrate the radiological findings of a wide spectrum of lung diseases, with emphasis on their similarities with the common presentations of COVID-19 pneumonia.   This type of injury has a non-specific appearance, ranging from nodular opacities to larger confluent zones of peripheral consolidation (7). Although it resembles COVID-19 infection in terms of appearance, appropriate clinical history-taking, together with rapid resolution of uncomplicated contusions in the follow-up (7), is helpful in establishing an accurate diagnosis.

Non-infectious causes:
Aspiration pneumonia, resulting from aspiration of different materials, causes various pulmonary complications, such as lobar and segmental pneumonia, bronchopneumonia, lung abscess, and empyema (11).

Chronic interstitial lung disease (ILD):
ILD is a major contributor to morbidity and mortality, associated with pulmonary diseases. They are ideally evaluated, using high-resolution computed tomography (HRCT) scan. Although imaging findings can be specific in some subgroups, some common radiological findings may be also found (Figures 8-11 COP can be considered as a differential diagnosis when detecting bilateral patchy consolidations with a peripheral distribution (16). This condition also clinically presents with fever, dyspnea, cough, and malaise, similar to COVID-19 pneumonia, and there is no significant response to common treatment protocols.     (Figures 12-14). Many of these conditions may present with alveolar hemorrhage, which radiologically appears as GGO zones. Evidence shows that a recent hemorrhage can ultimately lead to fibrotic changes (18). Moreover, multifocal, fluctuant, patchy, and nonsegmental consolidation, with no specific zonal predilection, is a common finding of Churg-Strauss syndrome (17,18). Airway involvement, including bronchial wall thickening, bronchiectasis, and pleural effusion, has been also reported in about 50% of these patients, which is much more common than the rate reported in cases of COVID-19 pneumonia (17). Air way involvement such as bronchial wall thickening and bronchiectasis may also be detectable.

Pulmonary emboli and pulmonary infarction:
Peripherally located ground-glass attenuation, commonly followed by segmental consolidation, is a feature of acute pulmonary emboli, accompanied by pulmonary infarction (18,19). The emergence of a triangular opacity with a broad base and a linear band, extending from the apex toward the hilum, has been reported as a radiological finding of pulmonary infarction. Moreover, a fan-shaped GGO can be a premonitory sign of this condition (19).
In some cases of pulmonary infarction, the focal area of GGO is surrounded by a complete or incomplete rim of consolidation, giving rise to the reversed halo sign (atoll sign) (Figure 15) (20). Pulmonary infarction was once recognized as a specific sign of COP, but later, it was identified in many other conditions, including pulmonary infections (20). This condition has been also reported in some cases of COVID-19 pneumonia. Nonetheless, the filling defect in the vessel supplying the abnormal zone must be identified for an accurate diagnosis. Generally, the main radiological finding of chronic pulmonary thromboembolism is mosaic attenuation, caused by decreased zonal perfusion and vascular constriction. A normally perfused lung may appear as hyperdense, relative to the oligemic zone, and may be misdiagnosed as a pathological GGO. This condition can be detected by observing the scarcity of vascular density in the low attenuation area and uniform attenuation changes in inspiratory and expiratory CT studies (18). Figure 16 demonstrates an example of approach to GGO in chest CT scan.

CONCLUSION
COVID-19 pneumonia is typically characterized by multifocal/multilobar zones of GGO, mostly in subpleural areas. The diagnosis of this condition is straightforward in the setting of coronavirus pandemic. However, radiologists should consider other possible differential diagnoses, especially when the clinical scenario is not in favor of viral pneumonia, the patient's RT-PCR result is negative, or there is an underlying disease. Figure 16. An example of a flowchart for the approach to GGO in chest CT scan.